1. Field of the Invention
The subject invention is directed to flow sensors and, more particularly, to flow sensors suitable for use in medication dispensing systems.
2. Description of the Prior Art
Over the years, various types of medication dispensing systems have been developed. Generally, these systems provide for delivery of medication from a variety of pressure chambers or pumps through a catheter and into the patient. Examples are shown and described in U.S. Pat. Nos. 3,951,147; 4,193,397; 4,360,019; and 4,373,527. Such systems are, however, subject to various failure modes such as a stalled pump condition, or an occlusion in the catheter. Such failures have been particularly troublesome where the delivery rate of medication is so low that the user is unaware of the interruption of medication flow until the physiological symptoms develop.
Accordingly, there was a need in the prior art for a device that would indicate the fluid flow condition in various types of medication delivery systems. Because of the potentially harmful consequences of interruptions in medication delivery, it was most important that such a flow sensor be reliable. However, such sensors would typically be applied to systems having at least some disposable components so that it was also important that the flow sensor could be inexpensively manufactured. Moreover, many such systems are carried by the patients on a daily basis in the normal course of their activities. These systems are compact and highly portable so that a compatible flow sensor would also need to be similarly compact and portable.
Various types of fluid pressure measuring devices were known in the prior art. For example, U.S. Pat. No. 4,398,542 is directed to a pressure measuring device wherein pressure of fluid flow between a flexible membrane and a body is sensed by a pressure transducer. Other examples of pressure measuring devices are shown in U.S. Pat. Nos. 4,185,641; 3,240,207; and 1,861,999. However, none of those devices afforded a suitably reliable yet inexpensive flow sensor that was sufficiently compact for use in a wide range of medication dispensing systems.
Some medication delivery systems have employed various schemes for providing audible or visual alarms in response to over-pressure conditions. However, these were pressure-sensitive arrangements that were not responsive to flow conditions. Indeed, they generally required a substantial pressure accumulation to activate the alarm. Moreover, the benefit of such alarms was seriously compromised for patients having visual or auricular disabilities.
Consequently, various methodologies for detecting medication flow were developed. For example, the user of a portable medication dispensing device would occasionally remove the needle from his body to observe whether medication was, in fact, flowing. Such methodologies were, at best, uncomfortable and awkward for the user. Moreover, they tended to expose the medication system to contamination and the patient to infection.
Accordingly, there was a need in the prior art for a flow sensor that was inexpensive, reliable, and suitable for use on medication infusion devices. Preferably, such a sensor would provide a tangibly perceptive indication of flow conditions.